In a typical curing press, a tire is cured in a mold cavity in which the green (unvulcanized) tire is biased against a metal tread ring by a curing bladder at elevated pressure and temperature, for a time sufficient to cure the tire. The tire is placed in the curing press so that the vertical axis coincides with the axis of rotation of the tire. In other words, the tire in the mold cavity lies in the horizontal plane. The mold cavity is provided with a tread ring which has generally horizontal and transverse (inclined to the horizontal) rib elements (ring ribs), forming circumferential ring ribs disposed on the inner circumferential surface of the tread ring. These rib elements project radially inwards, that is, towards the center of the mold cavity, for a predetermined depth which determines the depth of tread in the tire to be cured. The rib elements are in spaced-apart relationship which provides tread blocks of preselected size and pitch sequence in the tread design of the tire being cured.
It is essential to vent the air which would otherwise be trapped between the inner surface of the tread ring and the radially expanding surface of the green tire as it is being biased towards the tread ring. Heretofore, this air has been vented radially with radially extending passages (radial vents), as well as with vertically (relative to the plane in which the tire is being cured) extending vents, referred to as "cross-vents" in the prior art, and referred to as "arch-vents" herein. The vents place the mold cavity in open communication with an annular vent space between the outer surface of the tread ring and the inner surface of each mold half. A passage places the annular vent space in open communication with the atmosphere, thus venting trapped air.
In the typical configuration of radial vents, each vent has a conical shape, the base having a slightly larger cross section than the apex, so that upon curing of the tire, a cone of rubber (vent stub) is formed in the vent. The vent stub is of sufficient diameter so that, upon removal of the tire from the mold cavity by a vertically upward tire-stripping force, the base of each vent stub is large enough to pull the entire vent stub from the vent without allowing the vent stub to break off in the vent.
The slighter the taper of a vent stub, the more difficult it is to predict just where a break will occur. The larger the base of the vent stub, the greater its strength, and the less likely will it be that the vent stub will break off in the vent. Most breaks occur near the base, because of the initial shearing force at the inner surface of the tread ring, when the cured tire is stripped from the mold cavity. Since the upward force to strip a tire in a press, whether an AUTOFORM or BAG-O-MATIC type press, is essentially fixed by the stripping mechanism associated with the piston of each press, the problem was to provide a venting means (vent, or passage) with a configuration which would result in the formation of as inconspicuous and unobtrusive a narrow bridge (vent bridge) of rubber as possible, yet strip the narrow and small bridge so formed in a conventional curing press, without breaking the bridge at any point other than its narrowest and weakest point, preferably its mid-point.
The strength of a vent bridge, just prior to stripping the tire also depends upon the thermal history of the vent bridge, and in particular, the temperature gradient in the tread ring at that time, which gradient determines the temperature profile along the radial length of each side of the bridge. When the tire is stripped, the bridge is broken, resulting in the vent stubs which are to remain in the finished tire. Since this temperature profile is not the same for every bridge in the cured tire about to be stripped, the relative strengths of the bridges in different portions of the tread ring is not equal.
The effect of temperature on the hot cured elastomer makes it difficult to calculate the effect of the various forces on the sides of the bridge during stripping. Therefore the practical effects of shearing forces were measured by arduous trial and error. Since the force required to strip a tire from the mold is so large, the particular very small dimensions of each side of the vent bridge first seemed to be irrelevant with respect to where the bridge would break. Most particularly, it seemed that whether the force acted on a bridge end with a cylindrical cross-section, or whether that cross-section was rectangular, or any other geometry, would be immaterial.
Eventually, we were surprised to find that the direction of the shearing force was the overriding factor, and that the height of the stub (in the horizontal direction, measured along the radius of the tire) at the bridge's ends determines it strongest section, where it will not break, much more so than the width (thickness, measured in the vertical plane, that is, normal to the horizontal plane in which the mold lies).
It is self-evident that if there is a single vent for a tread block, any vent stub broken off in the vent so as to plug it, would negate venting the tread block formed behind the vent, resulting in blemishes or bubbles in the surface of the tread block. Therefore every vent stub, whether a radial vent stub or a cross-vent stub, must be pulled out of the vent passage without leaving a plug. Since each tread block on a tire must be vented, there is at least as large a number of vent stubs on a tire as there are tread blocks, and the probability of a broken vent stub begins to weigh against reliable and effective vent stub removal.
Even when conventional radial venting is effectively executed, with stubs of sufficient basal strength, the cured tire removed from the curing press must be destubbed in a later operation before the tire is cosmetically acceptable when sold. This necessary operation is both time-consuming, expensive and wasteful. The larger the diameter of the vent, the larger the vent stub, and the less visually appealing is the dressed surface of the tire; also, the more the waste of rubber, since cured vent stubs dressed from a cured tire, have no economic value.
The mounting economic pressure of the market place has, over the years, led tire mold designers to vent a tire without resorting to radial vents. Since it is essential that the circumferential surface of each tread block formed within the confines of the ring ribs, be smooth, that is conform exactly to the inner surface of the tire ring, they used "cross-vents" to avoid dressing the cured tire, and to minimize the waste of rubber. These cross-vents place the spaces above each of the tread blocks within the confines of ring ribs, in open communication with each other, so that air trapped in these spaces is progressively flowed towards the lateral circumferential center-line of the molds where the parting line affords escape to the atmosphere.
The difference between the design requirements of a radial vent stub and a cross-vent stub are similar only to the extent that each seeks to provide effective venting of trapped air with the smallest vent passage practical. The difference in the design requirements is that a cross-vent bridge must break at a predetermined location with just a single break, while radial vent stubs must not break at all. The cosmetic requirements of the marketplace require that radial vent stubs be removed, that is, cured tires must be de-stubbed, while cross-vent stubs are not removed provided they are relatively unobtrusive and maintain a smooth upper tread surface.
Cross-vents are drilled, cylindrical, slightly tapered passages which provide the necessary venting. The tread blocks in the cured tire are therefore bridged with a bridge having a narrowed mid-section, and, cylindrical, slightly tapered sides of cured rubber, just before the tire is stripped. When the tire is stripped from the mold, this bridge is broken at its weakest point. As with the radial vent stubs, the cross-vents have enlarged bases with a cylindrical cross-section, so as to provide a tapered cross-vent stub which will have sufficient basal strength to break the bridge without leaving a portion of it to plug the cross-vent. The weakest point in the bridge is determined not only by the geometry of the rubber bridge but by the thermal history of each side of the bridge.
Since the geometry is the controlling factor, whether in a radial vent stub or with a cross-vent bridge, the larger the base, the less likely was the cross-vent stub to break off near the base, or in more than one location in the bridge, so as to leave a plug in the cross-vent. However, since the broken cross-vent bridge is to be left in the finished tire to attempt to make the bridge-portions as unobtrusive as possible for cosmetic reasons, it was essential to keep the bases of the cross-vent stubs as small as possible. This resulted in accepting a greater risk of plugging the cross-vents. Predictably, with the exigencies of high quality control standards required to be maintained economically, the resulting frequency of plugging became unacceptable. The conical cross vents in tread rings currently used, have failed to solve the problem of plugging due to the cross-vent bridge breaking off unpredictably.
It is self-evident that the cross-vent bridge must break cleanly at a single break-point in the bridge, so as not to leave a portion to plug the vent. It is also self-evident that simply increasing the diameter of the vent passage may increase the force required to break each cross-vent bridge, but it becomes progressively less unobtrusive, and because there is little directional bias for the shear (force) on a cylindrical base, its enlarged diameter provides little assurance that more than one break will not occur, thus leaving a plug in the cross-vent.
The difficulty in coping with the problems of using cross-vents has resulted in tire mold designers opting to vent the mold cavity radially, with concealed tire vents, so as to minimize the de-stubbing required to finish a tire, as for example in U.S. Pat. Nos. 3,553,790 and 3,692,090 to Brobeck et al, and still more recently in U.S. Pat. No. 4,436,497 to Dahl et al. The unique "butterfly-shaped" or "double-wedge-shaped" design of our cross-vent, referred to herein as an "arch-vent", not only provides effective venting and leaves a finished tread surface free of radial vent stubs, but does so reliably and reproducibly, assuring that each arch-vent bridge will break at its weakest point, near its midpoint, and not leave a plugged arch vent when the tire is stripped. Moreover, arch-vent stubs provide improved traction in mud and snow, attributable to the stiffness imparted to the stubs because of their wedge shape; an advantage not shared by conical prior art cross-vent stubs.